1. Field of the Invention
The present invention relates to an antenna used in millimeter waves or microwaves, and more particularly, to abeam scanning antenna in which the direction of a beam of radiated or absorbed electromagnetic waves can be changed by changing the phase of the electromagnetic waves.
2. Description of the Related Art
In millimeter waves or microwaves applied radar technology is well-known such type radars that in order to accurately recognize the position of an object in a wide target area, the beam of electromagnetic waves is focused and thinned, while being scanned at the same time.
On the other hand, in recent years, a millimeter wave radar has been about to be mounted on cars so that obstacles are detected while cars are running. The millimeter wave radar is to be mounted on cars, and accordingly is required to be small in size, light in weight, high in reliability and low in cost.
Beam scanning antenna of such types that switching among a plurality of antennas is carried out by a PIN diode or the like and that the antenna itself is swung are put into actual use as beam scanning antenna for car radars.
Moreover, a beam scanning antenna using a phase shifter is used in many radars although not commercialized for car radars at the present time. In the beam scanning antenna, the direction of a beam radiated from an array antenna or inputted to the array antenna is changed by accurately changing the phase of a signal fed to each element of the array antenna by use of a phase shifter such as a latched ferrite.
The conventional beam scanning antennas, however, have the following problems:
First, high reliability as a radar to be mounted to a vibrating apparatus such as a car radar can hardly be ensured in the beam scanning antenna, which swings itself, because of its mechanical driving portion, although the beam scanning antenna is easily manufactured because of its simple structure and the beam direction can finely be switched. Further, reduction in size cannot be achieved because of space for swinging the antenna.
A beam scanning antenna of such a type that switching among a plurality of antennas is carried out has an advantage of high reliability because no mechanical control is used. This type beam scanning antenna, however, has a problem of low availability of each antenna because only part of the plurality of antenna are operated at a moment, and a problem that the reduction in size of the entirety of the beam scanning antenna is hardly achieved because of necessity of the plurality of antennas. Further, since an antenna aperture of a predetermined area or more is required of the beam scanning antenna of this type irrespective of the configuration and material of the antenna elements to obtain a desired antenna gain or beam diameter, it is necessary to provide the plurality of fixed antennas with the same property to obtain the desired antenna gain or beam diameter. Consequently, it cannot be helped that the overall antenna area is large. At the same time, a switch for high frequency applications suffers large insertion loss and is difficult to operate with a high degree of efficiency. Moreover, in order to finely switch the beam direction, the number of necessary antennas increases with the result that the overall area of the antennas increases and the antenna gain decreases with increases in number of switches. Consequently, it is practically impossible to provide a beam scanning antenna in which the beam direction can be finely switched.
The beam scanning antenna using a phase shifter has a limitation of use because the phase shifter is normally large in size and expensive.
As described above, presently, no beam scanning antenna meets the requirements of being small in size, light in weight, high in reliability and low in cost although the beam scanning antenna is a technology expected to be increasingly used, particularly, for car radars in the future.
The invention is made in view of the above-mentioned problems of the related art, and an object thereof is to provide a small-size, lightweight, high-reliability and low-cost beam scanning antenna having no mechanical driving portion and capable of scanning of an electromagnetic wave beam by only electric signals.
The invention relates to a beam scanning antenna comprising two conductor plates disposed parallel to each other; a primary radiator for transmitting and receiving electromagnetic waves; and a wave collector for electromagnetic waves, shaped like a flat plate, the primary radiator and the wave collector being disposed between the two conductive plates, a plurality of input and output portions for joining the electromagnetic waves between the wave collector and the input and output portions being disposed on one of the conductive plates, the wave collector including a substrate formed of a material whose dielectric constant can be changed by an electrostatic field, a plurality of strip-shaped electrodes disposed on one principal surface of the substrate so as to be substantially parallel to one another in a direction of travel of the electromagnetic waves, and a counter electrode formed on the other principal surface of the substrate, the counter electrode extending on a substantial entirety of the other principal surface or being separated into strips so as to be opposite to the plurality of strip-shaped electrodes formed on the one principal surface,
wherein the electrostatic field is applied between the electrodes formed on the one principal surface and the counter electrode formed on the other principal surface to partially change a dielectric constant of the wave collector in a direction perpendicular to the direction of travel of the electromagnetic waves, whereby a direction of a beam of the electromagnetic waves radiated or absorbed through the input and output portions is made variable.
Moreover, in the invention, it is preferable that within a plane perpendicular to the direction of travel of the electromagnetic waves in the substrate of the wave collector, the wave collector has a dielectric constant substantially unchanged in a direction perpendicular to a shorter side of the strip-shaped electrodes formed on the one principal surface, and has in a direction parallel to the shorter side, refractive indices for the electromagnetic waves which are distributed as a quadratic function of distance with a predetermined point as a peak.
Moreover, in the invention, it is preferable that the dielectric constant of the material of the substrate whose dielectric constant can be changed by the electrostatic field is changed by 20% or more by application of the electrostatic field, and a dielectric loss thereof is 1% or less.
Moreover, in the invention, it is preferable that the material of the substrate is made of a ferroelectric of (Ba, Sr)TiO3, BaTiO3 or SrTiO3, or a liquid crystal material such as nematic liquid crystal, cholesteric liquid crystal or smectic liquid crystal, or a liquid crystal polymer.
Moreover, in the invention, it is preferable that within a plane perpendicular to the direction of travel of the electromagnetic waves in the substrate of the wave collector, the wave collector has a highest dielectric constant at a center of the substrate in the direction parallel to a shorter side of the strip-shaped electrodes formed on the one principal surface and has dielectric constants which are reduced so that square roots of the dielectric constants decrease as a quadratic function of distance toward a periphery of the substrate, whereby the refractive indices for the electromagnetic waves are decreased as the quadratic function of distance with the center of the substrate as a peak.
The invention relates to a wave collector comprising: a substrate made of a material whose dielectric constant can be changed by an electrostatic field, in which substrate electromagnetic waves travel in a predetermined direction of travel; strip-shaped electrodes disposed on one principal surface of the substrate so as to be substantially parallel to one another in the direction of travel of the electromagnetic waves; and a counter electrode formed on the other principal surface of the substrate, the counter electrode extending on a substantial entirety of the other principal surface or being separated into strips so as to be opposite to the plurality of strip-shaped electrodes formed on the one principal surface,
wherein a dielectric constant of the wave collector in a direction perpendicular to the direction of travel is partially changed by applying an electrostatic field between the electrodes formed on the one principal surface and the counter electrode formed on the other principal surface.
According to the invention, a beam scanning antenna comprises two conductor plates disposed parallel to each other; a primary radiator for transmitting and receiving electromagnetic waves; and a wave collector for electromagnetic waves, shaped like a flat plate, the primary radiator and the wave collector being disposed between the two conductive plates, a plurality of input and output portions for joining the electromagnetic waves between the wave collector and the input and output portions being disposed on one of the conductive plates, the wave collector including a substrate formed of a material whose dielectric constant can be changed by an electrostatic field, a plurality of strip-shaped electrodes disposed on one principal surface of the substrate so as to be substantially parallel to one another in a direction of travel of the electromagnetic waves, and a counter electrode formed on the other principal surface of the substrate, the counter electrode extending on a substantial entirety of the other principal surface or being separated into strips so as to be opposite to the plurality of strip-shaped electrodes formed on the one principal surface,
wherein the electrostatic field is applied between the electrodes formed on the one principal surface and the counter electrode formed on the other principal surface to partially change a dielectric constant of the wave collector in a direction perpendicular to the direction of travel of the electromagnetic waves, whereby a direction of a beam of the electromagnetic waves radiated or absorbed through the input and output portions is made variable. Consequently, a beam scanning antenna having no mechanical driving portion inside and being small in size can be obtained, the reliability can be improved compared to the conventional case where the antenna itself is swung and the thickness can be reduced because it is unnecessary to secure a space for swinging the antenna, and the size as a radar can significantly be reduced compared to the conventional case where switching is made among a plurality of antennas. As a result, a lightweight, small-size, high-performance and low-cost beam scanning antenna optimum, for example, as a car radar can be provided.
As the material whose dielectric constant can be changed by the electrostatic field which material is used for the substrate of the wave collector in the beam scanning antenna of the invention, various kinds of materials may be used that are small in dielectric loss and whose dielectric constants are changed according to application of the electrostatic field in a frequency band such as microwaves or millimeter waves used in this antenna, specifically, a frequency band of approximately 3 to 80 GHz. As a characteristic of the material, since the beam scanning antenna is mounted on a car or the like, it is desirable that for application of a voltage of several tens of volts, the dielectric constant be changed by approximately 20% or more compared to a case where no electrostatic field is applied and that the dielectric loss be as small as not more than 1%.
Concrete examples of this material include ferroelectrics of (Ba, Sr)TiO3, BaTiO3, SrTiO3 and the like, liquid crystal materials such as nematic liquid crystal, cholesteric liquid crystal and smectic liquid crystal sealed in a cell made of a dielectric being small in dielectric loss in a millimeter wave region such as glass, and liquid crystal polymers such as BL-036 manufactured by Merk and Co., Vectra and Xydar.
As the shape and dimensions of the substrate made of this material, a substrate shape of a size substantially the same as that of the antenna array, specifically, 30 to 50 mmxc3x9760 to 80 mm in the case of a vehicle-to-vehicle distance radar is used.
Moreover, the basic idea of the structure in which the wave collector is formed by using the substrate made of the material, disposing a plurality of strip-shaped electrodes substantially parallel to one another in the direction of travel of the electromagnetic waves on one principal surface, disposing the counter electrode formed on the other principal surface of the substrate which counter electrode extends on a substantial entirety of the other principal surface or is separated to be opposite to the strip-shaped electrodes formed on the one principal surface, and partially changing the internal dielectric constant by applying the electrostatic field between the electrodes formed on the one principal surface and the counter electrode formed on the other principal surface is, for example, as follows:
As a lens that converts electromagnetic waves between spherical waves and plane waves which lens has heretofore been used as a wave collector in a beam scanning antenna, one formed in a so-called lens shape in which the outside shape of a dielectric having a uniform dielectric characteristic is surrounded by a part of a curved surface have been used most frequently. Moreover, in some beam scanning antennas, a so-called rod lens has been used in which the compositions of the center and the periphery of the rod are continuously changed so that the refractive index for electromagnetic waves is highest at the center and decreases as a quadratic function with respect to the diameter toward the periphery. By setting the dielectric constant so as to be highest at the center of the flat substrate and to decrease so that the square root of the dielectric constant decreases as a quadratic function of the distance toward the periphery like in the rod lens, the refractive index for electromagnetic waves decreases as a quadratic function according to the distance, so that even a substrate being rectangular in outside shape can be caused to operate as a wave collector like the lens made of a material having a uniform dielectric characteristic.
Therefore, by using a substrate made of a material whose dielectric constant is changed according to the intensity of the applied electrostatic field and constructing an electrode structure such that a plurality of strip-shaped electrodes substantially parallel to one another in the direction of travel of the electromagnetic waves is disposed on both surfaces so as to be opposed to each other or that a plurality of strip-shaped electrodes substantially parallel to one another in the direction of travel of the electromagnetic waves is disposed on one surface and a counter electrode that becomes the ground plate is disposed on the entirety of the other surface so as to be opposed to the strip-shaped electrodes, the dielectric constant in the substrate of the wave collector can be distributed substantially in an arbitrary condition in the range of the dielectric characteristic of the material so that the direction of the beam of the electromagnetic waves can be moved between the primary radiator and the input and output portions.
Moreover, in the beam scanning antenna of the invention, by adjusting the external electric field (electrostatic field) applied between the strip-shaped electrodes and the counter electrode so that in the substrate of the wave collector, within the plane perpendicular to the direction of travel of the electromagnetic waves, the dielectric constant is substantially unchanged in the direction perpendicular to the shorter side of the strip-shaped electrodes on the one principal surface and in the direction parallel to the shorter side, the refractive index for the electromagnetic waves, that is, the square root of the dielectric constant is distributed as a quadratic function of the distance with the predetermined point as the peak, a flat rectangular substrate can be caused to function as a plano lens, so that the substrate can efficiently be caused to operate as a small-size wave collector.
To distribute the square root of the dielectric constant as a quadratic function as mentioned above, as the electrostatic field applied to the strip-shaped electrodes of the wave collector, for example, a desired voltage obtained from a relational expression of the change of the dielectric constant and the voltage of the electrostatic field with respect to the counter electrode is applied to the strip-shaped electrodes. For example, in the case of a material whose dielectric: constant decreases linearly with respect to the applied electric field, adjustment is made so that a voltage of 0 V is applied to the central strip-shaped electrode, that a maximum voltage determined by the electrostatic breakdown voltage of the dielectric or the capability of the control circuit is applied to the outermost strip-shaped electrodes, and that the voltages applied to the strip-shaped electrodes therebetween are proportionally distributed according to the biquadrate of the distance between the center of each strip-shaped electrode and the center of the wave collector.
Moreover, according to the beam scanning antenna of the invention, since in the wave collector, the distribution of the internal dielectric constant can be controlled by controlling the electrostatic field externally applied to the strip-shaped electrodes and the counter electrode by voltage control or the like, it is easy to parallelly shift the dielectric constant distribution in the wave collector rightward or leftward with the position of the substrate, that is, the position of the wave collector fixed by changing the externally applied electrostatic field, so that the position of the focal point of the wave collector with respect to the primary radiator can be moved easily. Consequently, the structure can be caused to operate as a beam scanning antenna by changing the beam direction.
The dimensions of the strip-shaped electrodes and the counter electrode to which electrodes such an electrostatic field is applied are set to be as small as possible so long as the processing method and the costs permit. Moreover, these electrodes are formed on the principal surfaces of the substrate by shaping electrodes of a metal such as copper or aluminum into a desired pattern configuration and dimensions by etching. It is to be noted that the electrodes may be formed by a lower-cost method such as the thick film method when the material permits.
Moreover, it is desirable in simplification of the structure that the two metal plates disposed parallel to each other which plates constitute the beam scanning antenna of the invention be made of copper or aluminum plates and disposed parallel to each other with a distance the same as the thickness of the primary radiator in between. These two metal plates are also used as members constituting the housing of the beams canning antenna.
Moreover, as the primary radiator transmitting and receiving electromagnetic waves, one is used that is capable of efficiently radiating an electromagnetic beam such as a wave guide with open ends or a dipole antenna. The primary radiator is situated in the position of the focal point of the wave collector and disposed between the two metal plates.
Further, as the input and output portions joining the electromagnetic waves between the wave collector and the input and output portions, slots or the like are used that are formed in a shape where the electromagnetic waves transmitted between the parallel plates are not reflected at the input and output portions. It is desirable that the input and output portions be disposed on one of the two metal plates in positions as close to the wave collector as possible.
According to the beam scanning antenna of the invention, the primary radiator transmitting and receiving electromagnetic waves and the flat wave collector for the electromagnetic waves are disposed between the two conductive plates, for example, metal plates disposed parallel to each other, and a plurality of input and output portions for joining the electromagnetic waves between the wave collector and the input and output portions is disposed on one of the metal plates; in the wave collector, a plurality of strip-shaped electrodes substantially parallel to one another in the direction of travel of the electromagnetic waves is disposed on one principal surface of the substrate made of the material whose dielectric constant can be changed by the electrostatic field, and on the other principal surface, the counter electrode is formed on the other principal surface of the substrate so as to extend on a substantial entirety of the surface or to be shaped like a strip and is disposed so as to be opposed to the strip-shaped electrodes on the one principal surface; and by applying the electrostatic field between the strip-shaped electrodes and the counter electrode to thereby partially change the dielectric constant of the wave collector in the direction perpendicular to the direction of travel of the electromagnetic waves, the direction of the beam of the electromagnetic waves radiated or absorbed through the input and output portions can be changed. Consequently, a beam scanning antenna having no mechanical driving portion inside and being small in size can be obtained, the reliability of the operation can be improved and the thickness can be reduced, so that the size as a radar can be reduced significantly. As a result, a lightweight, small-size, high-performance and low-cost beam scanning antenna optimum, for example, as a car radar can be provided.